Recently, with the increase of the degree of integration and smaller sizing of semiconductor elements, the packaging (encapsulating) technology of semiconductor elements in the field of semiconductor devices becomes more and more complicated. Hitherto, a plastic package for encapsulating a semiconductor element using a cured resin has actively used due to the excellent productivity and the low cost. In the plastic package, a dual inline package (DIP) has been widely used but recently in addition to DIP, a surface mount package such as a plastic leaded chip carrier (PLCC), a quad flat package (QFP), a thin quad flat package (TQFP), a thin small outline package (TSOP), etc., have also been used. Furthermore, from the remarkable improvement of the recent input/output (I/O) electric characteristics and the ease of the packaging technology, a so-called one-surface encapsulation is being watched with keen interest and has been practically used in some semiconductor devices.
A practical embodiment of the one-surface encapsulation is a semiconductor device of a packaged state usually called BGA (ball grid array) as shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, in a semiconductor device 1, a semiconductor element 3 is loaded on a substrate 4 and the semiconductor element 3 is encapsulated with a cured resin 2. The encapsulation with the cured resin 2 is an encapsulation of only the side of the substrate 4 loading thereon the semiconductor element (one surface encapsulation). On the surface of the opposite side of the substrate to the encapsulated side are formed soldering terminals 5 each having a nearly spherical form.
Another embodiment of the one-surface encapsulation is a semiconductor device having formed therein a heat sink 12 as shown in FIG. 2. The heat sink 12is for radiating the heat generated from the semiconductor element 3. As shown in FIG. 2, the heat sink 12 is disposed at a definite portion of a substrate 4a and the semiconductor element 3 is loaded on the substrate 4a such that the semiconductor element 3 is located on the heat sink 12. The semiconductor element 3 is encapsulated with a cured resin 2 and the encapsulation is also a one-surface encapsulation for only the side of the substrate 4a on which the semiconductor element 3 is loaded as the case described above.
As described above, the semiconductor devices each formed by encapsulating a semiconductor element with a cured resin are excellent in the mass production and can be produced at low cost. Further, the semiconductor devices formed by the above-described one-surface encapsulation can give a high performance by increasing the degree of integration of semiconductor elements. However, plastic packaged semiconductor devices using a conventional cured resin are insufficient in the thermal cycle test (TCT) characteristics and a cracking resistance at immersing in a molten solder (hereinafter referred to as "cracking resistance") as compared with ceramic packaged semiconductor devices. Further, in the plastic packaged semiconductor devices, a problem arises that the semiconductor device is warped. That is, when producing the plastic packaged semiconductor device, after the encapsulation step with a cured resin, the encapsulated semiconductor is treated by various heat-treatment steps from room temperature to a reflow soldering step. In this case, since the cured resin, the substrate, etc., each differs in the properties such as the linear expansion coefficient, etc., the semiconductor device warps to the side having a low linear expansion coefficient. In the semiconductor device that such a warp occurred, since gaps form between parts such as the cured resin, the substrate, the semiconductor element, etc., water permeates into the inside of the semiconductor device through the gaps to greatly reduce the reliability of the device. In particular, the problem of the warp is serious in the one-surface encapsulated semiconductor device and the semiconductor device having the heat sink described above and even when the performance of the semiconductor device is improved, the fact is that the reliability is not improved.